Significant improvements in the performance of production automobiles and other vehicles have been achieved by the provision of hydraulic mounts for the dampening of shocks and/or other vibrations. Standard hydraulic mounts used for these and other applications typically have interconnected variable volume chambers between which hydraulic fluid passes during excitation of the mount. One or both of the variable volume chambers is bounded by a compliant member which functions to carry the static and dynamic load on the mount and, also functions as a piston to "pump" the hydraulic fluid between the opposing chambers. Resistance of the fluid to flow between the chambers opposes and damps vibratory and similar forces imposed upon the mount, and in conjunction with other factors, provides for reliable and precise dynamic stiffness characteristics.
More versatile fluid mounts have been developed which utilize fluid inertia forces to achieve and/or to enhance the desired attenuation of vibratory forces. So-called "inertia augmented" damping is provided by hydraulic mounts which include a tube or "inertia track" which confines and directs fluid between the upper and lower variable volume chambers of the mount. Oscillation of the liquid within the inertia track offers an "inertence" or mass-like resistance to the compliant member pumping forces, which is in phase with input disturbance displacement and opposite in direction to input acceleration. The inertial forces of the oscillating fluid reduce the dynamic stiffness of the mount at some particular frequency of input excitation Illustratively, a plot of the dynamic stiffness against the excitation frequency of mounts of the fluid inertia type typically include a "notch-like" region at which the dynamic stiffness of the mount is greatly reduced and may be considerably less than its static stiffness, followed by a "peak" of large dynamic stiffness (FIG. 1). A mount may be so designed to cause the foregoing abrupt reduction in its dynamic stiffness (hereinafter referred to as the low stiffness "notch") to occur at a particular excitation frequency f.sub.o where a particular vibration problem exists. For example, in many automotive applications, excessive mount stiffness can result in transmission of vibration to the automobile frame and vehicle interior. Such objectionable vibration may be caused by engine firing vibrations generated at a particular engine speed and may be substantially eliminated by the use of an inertia type engine mount that is specifically designed so as to possess its minimum or low stiffness notch at the frequency of the aforesaid vibrations.
While the predesignated selection of the low stiffness notch for an inertia type mount may be adequate for the isolation of input excitations occurring at one particular frequency, problem vibrations such as those producing undesirable vehicle vibrations may occur at a number of significantly different engine speeds and mount excitation frequencies. It is therefore desirable for an inertia type mount to be readily adjustable or dynamically tunable, so as to permit selective variation during mount operation of the frequencies at which the mount has very low dynamic stiffness.
A well accepted method for providing an inertia type mount which is to some extent dynamically tunable is to provide a means for varying the flow path cross sectional area (A) of the inertia track. The input frequency at which the low stiffness notch occurs, and the "depth" of the notch is a function of, among other things, the fluid inertia (I.sub.f) and the fluid resistance in the inertia track (R.sub.f). Since the magnitude of the inertia which produces the low stiffness notch varies as a function of the cross-sectional area of the inertia track, immediate and controllable adjustments to the fluid flow cross-sectional area through the inertia track can be used to shift the notch on the dynamic stiffness curve to a desired frequency. Known mounts of this type provide for relatively complicated valves having a variable orifice size. Other devices contain a plurality of flow passageways between the chambers, which function in parallel and are selectively opened or closed to vary the overall cross sectional flow area between the chambers. For example, see U.S. Pat. No. 4,641,808 and U.S. Pat. No 4,733,758.
A shortcoming associated with dynamically tuned, inertia type fluid mounts of the foregoing type is that large resistances to fluid flow generated within the inertia track caused by a substantial reduction in the flow path cross sectional area can greatly diminish the depth of, if not altogether eliminate, the low stiffness notch and thus its effects upon the mount operating characteristics. The depth of the notch is a strong function of the resistance to fluid flow divided by the fluid inertia within the inertia track (R.sub.f /I.sub.f), such that a low value for (R.sub.f /I.sub.f) will produce a low notch stiffness and, thus, a preferred "deeper" notch. It is accepted that resistance to fluid flow (R.sub.f) in an inertia track is proportional to one over the square of the area (1/A.sup.2), while the inertia is proportional to one over the area (1/A). Accordingly, R.sub.f /I.sub.f will significantly increase (adversely diminishing the notch depth) as the cross sectional area (A) is made smaller. The notch depth is therefore very sensitive to changes in the cross sectional area of the inertia track because of the second order relationship between the cross-sectional area and resistance.
In contrast, the length of the inertia track (L) is proportional both to the inertia (I.sub.f) and resistance (R.sub.f) such that changes in the length do not produce the same adverse effect on the value of R.sub.f /I.sub.f. Since the resistance and inertia within the inertia track both vary linearly with its length, immediate and controlled adjustments to the length thereof would not as dramatically affect the depth of the notch (that is, increase the dynamic stiffness at the notch or notch stiffness) to the same extent as adjustments to the flow path cross sectional area. For this reason a mount in which the length of the inertia track is adjusted as opposed to the cross sectional area in order to change the mount operating frequency at which the notch occurs will have superior performance over a wider range of frequency values.
It may, therefore, be desirable to dynamically tune an inertia type mount by adjustment of the length of the inertia track instead of its cross sectional area in order to select the frequency at which the low stiffness notch can be utilized to avoid undesirable vibrations.